Engineering Plastics Get Tough, Lightweight

Engineering plastics continue to be designed into more and more parts and components for a broad variety of applications. Their versatility, durability, light weight, and easy customization make them appealing to engineers for solving design problems in a number of industries, from tiny medical devices to huge offshore oilrigs.

A recent study by market research firm Ceresana revealed that the three largest categories of global demand are coming from consumer goods, electronics and electrical uses, and the transportation industry. The first two categories combined represent nearly 25 percent of overall demand.

These injection-molded, high-precision plastic shafts and gears were made for a two-stage reduction transmission used in automotive power lift gates. The first-stage gear and shaft (far left and left) and second-stage output plastic gear (right) are injection molded from Celcon acetal copolymer (POM) M90 and Celcon GC25T, respectively. The second-stage output shaft (far right) is injection molded from Celstran PA 66-GF50-02.

Consumer electronics is definitely among the fast-growing application areas for engineering thermoplastics, says Matthew Gray, director of consumer electronics marketing for SABIC's Innovative Plastics business. The continuing trend toward smaller, lighter, yet stronger devices has meant that engineering thermoplastics -- including those derived from biological, petroleum, and recycled sources -- are all in demand. Which type is chosen depends on customer preference.

One way we help them with their closed-loop recycling programs is taking recovered materials from recycled copiers and printers and converting them into raw materials used to manufacture new machines. Other customers like to use materials that contain post-consumer or post-industrial content. We have several options within the VALOX iQ resin and XENOY iQ resin families to meet these needs. Some grades provide up to 65 percent recycled content.

VALOX iQ is a PBT (polybutylene terephthalate) containing some post-consumer PET (polyethylene terephthalate) feedstock, with a 50 to 85 percent lower carbon footprint and about 10 to 15 percent higher flow than standard PBT resins. XENOY iQ is a blend of PBT or PET with PC (polycarbonate), known for its chemical, impact, and heat resistance.

Several of SABIC’s customers prefer materials derived from bio-based sources. For example, certain grades of its LEXAN HFD copolymer resin contain up to 10 percent castor oil, while still delivering high-flow ductility, impact resistance even at low temperatures, and the ability to do in-mold decoration.

Most of what’s driving these demands in consumer electronics centers on three basic trends: the continued miniaturization of devices, a growing consumer emphasis on aesthetics, and innovation in wearable technology. Says Gray:

Although devices are getting smaller, customers also want them to be increasingly multi-purpose. As a result, engineering thermoplastics must do more with less. For example, dielectric and heat conductive thermoplastics compounds enable smartphone and tablet OEMs to consolidate parts and integrate the antenna into a device’s framework.

The resulting integration produces a thinner, more lightweight phone, and the saved space creates new design options.

As always, you provide a comprehensive look at some of the cutting edge in materials, Ann. Lightweight engineering plastics are particularly interesting because as you show, they have such a broad range of application. I'm especially interested in their use as lightweight materials for solar-powered vehicles and medical devices.

Thanks, Elizabeth. Since plastics are, in effect, always a custom mix, the resins can be made to fit a wide variety of spec combinations. But I was a bit surprised at the mention of solar-powered vehicle applications.

Being a car guy, I've had bad experiences over the years with "plastic/composite" parts failing in automobiles. One of the worst for me were the timing chain gears that GM used in the small block chevrolet V8 engines in the 1970s. But I've experienced plenty of smaller failures in plastics that just don't hold up over the years in the rough circumstances of the vehicular world. Silly little things like clips for hoses and wire assemblies are frequent failures, but there are bigger problems too.

Just last month I finally upgraded the plastic gears in my Trans-Am's headlight motors to a brass gear. The design of the headlight motor features an electric motor with a metal worm gear, that meshes with a plastic ring gear that's connected to the shaft which turns the headlight motor. Instead of incororating discrete limit switches in the design, they made a headlight controller which senses the high current spike when the headlight has reached the stops and can't spin any more. When it detects that high current, it turns off the motor. Unfortunately, that metal worm gear is placing a lot of pressure against that plastic rings gear and it eventually breaks teeth off of the plastic gear. There's a cheap fix to flip the assembly 180 degrees and use the other side of the gear (since the rotation only uses half of the gear), but eventually teeth on both sides break, causing the headlight to make a grinding sound when it reaches the limit. (The controller doesn't sense the current spike but has a failsafe to shut the motor off after a few seconds.)

Luckily, there are companies who machine nice metal gears for this application, but they are a bit expensive. Since my Trans-Am was my first new car which I'm keeping to pass down to my son some day, I finally invested in the new brass gears and they work well.

Thanks for sharing your actual experience with what we write about, Jim_E. I'm sorry to hear that about the plastic components in your cars. Much of the problem here, or elsewhere, is due to incorrect spec-ing of materials, sometimes because of engineers but often, as we hear a lot, because engineers spec the right material but management doesn't like the fact that quality costs more. That said, I'm surprised brass gears are OK, especially in a car. My bad experience with them is in a coffee-grinder: they wore down way too fast, changing the grind to very coarse by default.

Your Story of the headlight worm gear sounds familiar. Historically, plastic gears of any resin (often nylon) just didn't have the life-span that brass gears can offer. Under the hood environments are abusive, experiencing vibration, heat, dirt, and chemical spills. It's easy to understand why the Automotive OEMs would choose injection molded gears over brass, at a fraction of the cost to produce the parts. They don't have to last forever-- only 3 years or 36,000 miles!

Maybe Ann's new examples of the VALOX PBT and the XENOY blends will change all that, and low-cost injection molded gears can get a new reputation for longevity; starting today.

As a P.S., here are some new engineering plastics from DuPont specifically for car applications that need resistance to high temperatures and chemicals: http://www.designnews.com/document.asp?doc_id=269020

Ann, I think that you will find that a whole lot of automotive "engineering" winds up being done by purchasing people who get rewarded for cutting costs, and it seems that they are awarded by suppliers for delivering POs. The evidence in that area is more circumstantial, such as purchasing people sporting diamond encrusted gold Rolex watches.

Sometimes a plastic part that can easily handle the calculated average loads is just not up to handling those larger occasional loads, at which point the failure is permanent even if the part sort of works after the damage. Purchasing people are great ones for cutting safety margins in order to reduce costs. But the result is much lower quality. But that reflects on the engineers and so purchasing does not care about reducing quality.

William, I know what you mean. That's true for a lot of industries, not just automotive. A long time ago I wrote for an electronics purchasing publication, the old EBN, and I spent most of my time trying to educate them on what engineers (mostly) already knew, as well as on what they were learning about new technologies. But the whole concept of how they buy what they buy is different, and that becomes a crucial factor in quality.

Years ago at one employer I had a stamp for my released drawings that stated "This system will not function as required unless it is wired according to the circuit shown". I needed to add , " and built with the specified materials", but at that job the problem was panelmen who took shortcuts. BUT the principle is identical.

University of Southampton researchers have come up with a way to 3D print transparent optical fibers like those used in fiber-optic telecommunications cables, potentially boosting frequency and reducing loss.

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